A bolt-nut unit is widely employed as a fastener in the machinery. In such a unit, a bolt and nut are threaded to each other to fasten objects. The fastened objects using the bolt and nut, however, may receive vibrations and/or shocks for a long time, so that the nut may loose from the bolt.
One general approach to prevent such loosing is to use a spring washer. The spring washer is a coil-shaped washer having a discontinuous cut portion and a twisted structure. The spring washer is elastically deformed when fastened between a bolt and nut with the nut being tightly threaded onto the bolt. Such elastic deformation of the spring washer may cause an elastic restoring force in a direction so as to increase a friction force between threads of the bolt and nut. Thus, the nut may be prevented from loosing from the bolt.
In addition to the spring washer, there have been employed a variety of other loosing prevention mechanisms to prevent the nut from loosing from the bolt.